The redacted version of the report does not state that there was any causal link between switching to SVTOL and the failure of the HMD. Only that the HMD happened to fail shortly after he switched modes:

    95. Approaching the final approach fix, the MFL made the radio call
    to convert the flight to Mode 4 (STOVL) and slow the flight to 150
    knots in preparation for the 100-knot slow landing. This was the last
    radio call Swede 12 heard from Swede 11. [Encl (17), (19)]

    98. MP reported his HMD flickered shortly after converting. Near
    simultaneously, a momentary caution displayed in his HMD that he
    perceived to be engine-related and then the HMD flickered out. While
    MP considered missed approach options, the HMD came back. [Encl (17)]

And from the evaluation:

    19. Primary factors contributing to the mishap can be traced back to
                    event that occurred at 13:32:05. This event induced
    failures of both primary radios,                       the TACAN, and
    the ILS. [FF 100, 101, 126-129]

    20. Additionally, it is probable that the HMD and PCDs were not
    operational for at least three distinct periods. [FF 98, 103, 104,
    106, 114, 132-136, 141]